High quality oscillators with long-term reliability are needed to ensure the interference-free transmission of digital messages. Quartz oscillators are generally used for such purposes, which are tuned by a microprocessor and a digital/analog converter, and form part of a phase control circuit, e.g., a basic phase-locked loop, see W. Ernst, Hartmann H. L.: New clock generators for EWSD, telecom report 9 (1986), brochure 4, pages 263-269.
Fairly expensive circuits are needed to fulfill the high accuracy and stability requirements of a clock generator, for example microprocessor-controlled digital phase control circuits (DPLLs) are used. Still, the physical characteristics of a controllable quartz oscillator set certain limits. Thus, a compromise must always be found between control range ("pulling" range of voltage-Controlled Crystal Oscillator (VCXO)) and stability, because the stability of the quartz oscillator decreases as the control range increases, thereby limiting the control range. Because of the nonlinearity of the control curve (voltage/frequency characteristic of "Kv"), which describes the function of the control voltage vs. frequency, tolerances of .+-.50% must be taken into account when the circuit is constructed. Therefore, the phase control circuit must be designed for amplification fluctuations (variations of "Kv" value) of .+-.50%, with a bandwidth tolerance of .+-.50%. Finally, the resolution of the control range is limited by the step width of the digital/analog converter, where temperature changes and nonlinearities are additional variables in the operation of the digital/analog converter. As a rule, digital/analog converters must be supplied with at least .+-.12 V of operating voltage. In tunable oven-controlled quartz oscillators, low long-term stability, temperature dependence and nonlinearity of the control curve are characteristics that result in problems with the creation of highly accurate clock generators.